Today, we find
many moons. The University of Houston's College of
Engineering presents this series about the machines
that make our civilization run, and the people
whose ingenuity created them.

This August of 2003, Mars
looms over our house like a lantern — the closest
it's been to Earth in sixty thousand years. Some
people look at it like some astral omen; and even I
feel a vestigial crease of worry that it might fall
through my roof.

But, near as it is, your telescope won't pick up
its moons, for they are very small. Yet there are
two of them. As we move outward from the Sun, we
begin with moon-poor planets. Mercury and Venus
have none. We have only one (but, I must say, ours
is large, lush, and glorious to see.)

Mars' moon Phobos is probably a captured asteroid.
It's oblong, and about six by nine miles in size.
Deimos is even smaller, and about four miles in
diameter. Both are so small that they weren't
discovered until 1877.

The last planet, tiny
Pluto, is three billion miles out from the Sun,
and it has only one moon — Charon, named
after the boatman on the River Styx. Charon is half
as large as Pluto itself. Indeed, Pluto might be
thought of as a double planet orbiting itself.

However, we know of 124 moons that
orbit Jupiter, Saturn, Uranus, and Neptune. We've found most of
those only since 1997, using new digital cameras
that can track very faint objects.

Galileo discovered the four largest of Jupiter's 61
moons in 1610. They're all comparable to our moon
in size. It took three centuries to find the next
largest one, because the rest of them range from a
hundred miles in diameter down to less than
one mile. Now the whole matter of what a
moon is has shifted under our feet.

Our solar system is littered with small objects.
Some orbit planets, but most orbit the sun. And
here we need the concept of a Hill Sphere.
That's an imaginary sphere surrounding a planet.
Within it, a moon will orbit stably. Outside it,
the gravitational pull of the planet is too small,
and that of the Sun is too large. Then the Sun will
destabilize the moon's motion and take it away from
the planet. Jupiter is so immense, and so far from
the Sun, that it has a huge Hill Sphere.

While that large sphere is the reason that Jupiter
and Saturn hold on to so many moons, it doesn't
tell us how those moons got there in the first
place. If a fast-moving satellite or comet passes a
planet, the planet needs some means for grasping it, and
slowing it down to just the right orbital speed.

The New York Times now reports an idea
that's just taking shape. It is that those big
planets must've once had far larger atmospheres —
enough to exert drag, and haul passing bodies into
orbit. But how they first created, and then lost,
those atmospheres, remains under debate.

And so I look over my house at red Mars with its tiny
unseen moons, and I realize how much mystery still
shrouds space — even so close as our own solar
system, even right here in my own back yard.

I'm John Lienhard, at the University of Houston,
where we're interested in the way inventive minds
work.